Laser cutting and machining method for plated steel plate, laser cut-and-machined product, thermal cutting and machining method, thermal cut-and-machined product, surface-treated steel plate, laser cutting method, and laser machining head

ABSTRACT

A laser cut-and-machined product made from a plated steel plate. A cut face of the plated steel plate is coated with plating-layer-containing metal of a top surface of the plated steel plate that is melted and/or evaporated at the time of laser cutting and machining.

This is a divisional application of pending U.S. application Ser. No.16/301,670, filed Nov. 14, 2018, which is a National Stage Entry ofPCT/JP2017/018528, filed May 17, 2017, which claims the benefit ofJapanese Patent Application No. 2017-095393, filed May 12, 2017,Japanese Patent Application No. 2016-099867, filed May 18, 2016 andJapanese Patent Application No. 2016-099292, filed May 18, 2016. Theentire contents of each of the above-identified documents, including thespecification, drawings and claims, is incorporated herein by referencein its entirety.

TECHNICAL FIELD

The present invention relates to a laser cutting and machining methodfor a plated steel plate, a laser cut-and-machined product, a thermalcutting and machining method, a thermal cut-and-machined product, asurface-treated steel plate, a laser cutting method, and a lasermachining head. More precisely, it relates to a laser cutting andmachining method for a plated steel plate, a laser cut-and-machinedproduct, a thermal cutting and machining method, a thermalcut-and-machined product, a surface-treated steel plate, a laser cuttingmethod, and a laser machining head in which when a plated steel plate islaser cut and machined, a laser beam is emitted to melt and/or evaporateplating-layer-containing metal of atop surface of the plated steelplate, and with an assist gas, the melted and/or evaporatedplating-layer-containing metal is guided toward a cut face, therebycoating the cut face with the melted and/or evaporatedplating-layer-containing metal, as well as relating to a lasercut-and-machined product, a thermal cutting and machining method, athermal cut-and-machined product, a surface-treated steel plate, a lasercutting method, and a laser machining head.

BACKGROUND ART

Conventionally, a work such as a plated steel plate is laser cut andmachined after removing a plated surface of the work (for example, referto Patent Literature 1).

According to a configuration written in the Patent Literature 1, aplated surface of a work is removed, and thereafter, the work is cut andmachined with a laser. This raises a problem in an improvement ofefficiency of the work laser cutting and machining. According to theconfiguration mentioned in the Patent Literature 1, a cut face of thework after laser cutting is not coated with plating-layer-containingmetal, and therefore, there is a problem that the cut face needs aproper surface treatment such as rustproofing.

PRIOR ART DOCUMENT Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application PublicationNo. H7-236984

SUMMARY OF THE INVENTION

Subjects to be solved by the present invention include a laser cuttingand machining method that carries out laser cutting and machining on aplated steel plate such that melted and/or evaporatedplating-layer-containing metal of a top surface of the plated steelplate flows toward and coat a cut face of the plated steel plate, aswell as a laser cut-and-machined product.

In order to resolve the above-mentioned problems, the present inventionprovides a laser cutting and machining method for a plated steel plate.The method carries out laser cutting and machining by irradiating a topsurface of the plated steel plate with a laser beam. At this time, themethod jets an assist gas to a laser cutting part of the plated steelplate to guide plating-layer-containing metal of the top surface meltedand/or evaporated by the laser beam toward a cut face of the platedsteel plate so that the cut face is coated with theplating-layer-containing metal.

According to the laser cutting and machining method for a plated steelplate, a focal position of the laser beam is adjusted within a range of+0.5 mm to −4.5 mm.

According to the laser cutting and machining method for a plated steelplate, a nozzle gap between a nozzle of a laser machining head and thetop surface of the plated steel plate is adjusted within a range of 0.3mm to 1.0 mm and an assist gas pressure within a range of 0.5 MPa to 1.2MPa.

According to the laser cutting and machining method for a plated steelplate, a laser cutting and machining speed is adjusted within a range of1000 mm/min to 5000 mm/min.

According to the laser cutting and machining method for a plated steelplate, a diameter of an assist gas jetting nozzle is 2.0 mm to 7.0 mm.

According to the laser cutting and machining method for a plated steelplate, the assist gas is a nitrogen gas or a mixture of 96% or highernitrogen gas and 4% or lower oxygen gas.

According to the laser cutting and machining method for a plated steelplate, a plate thickness is 2.3 mm, a plating quantity is K14, a nozzlediameter is 2.0 mm to 7.0 mm, an assist gas pressure is 0.5 to 0.9(MPa), and a cutting speed is 3000 to 5000 (mm/min).

According to the laser cutting and machining method for a plated steelplate, a plate thickness is 2.3 mm, a plating quantity is K27 or K35, anozzle diameter is 2.0 mm to 7.0 mm, an assist gas pressure is 0.5 to0.9 (MPa), and a cutting speed is 3000 to 5000 (mm/min).

According to the laser cutting and machining method for a plated steelplate, a plate thickness is 3.2 mm, a plating quantity is K27 or K35, anozzle diameter is 7.0 mm, an assist gas pressure is 0.5 to 0.9 (MPa),and a cutting speed is 2000 to 3000 (mm/min).

According to the laser cutting and machining method for a plated steelplate, a plate thickness is 4.5 mm, a plating quantity is K27 or K35, anozzle diameter is 7.0 mm, an assist gas pressure is 0.7 to 0.9 (MPa),and a cutting speed is 1500 to 2000 (mm/min).

The present invention also provides a laser cut-and-machined productmade from a plated steel plate, characterized in that a cut face of theplated steel plate is coated with plating-layer-containing metal of atop surface of the plated steel plate melted and/or evaporated at thetime of laser cutting and machining.

According to the laser cut-and-machined product, a plating thicknessaround an upper edge of the cut face is thinner than a plating thicknessat a position away from the cut face.

According to the laser cut-and-machined product, a plating melting rangeis within a range of 0.27 mm to 0.5 mm from the cut face.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a configuration explanatory diagram schematically showing aconfiguration of a laser cutting and machining apparatus according to anembodiment of the present invention.

FIG. 2 shows EPMA analysis results of cut faces cut by oxygen cutting,clean cutting, and easy cutting.

FIG. 3 shows enlarged photos of cut faces cut by clean cutting, oxygencutting, and easy cutting.

FIG. 4 shows enlarged photos of coating states ofplating-layer-containing metal on cut faces under different lasercutting and machining conditions.

FIG. 5 shows enlarged photos of coating states ofplating-layer-containing metal on cut faces under different lasercutting and machining conditions.

FIG. 6 shows enlarged photos of coating states ofplating-layer-containing metal on cut faces under different lasercutting and machining conditions.

FIG. 7 shows enlarged photos of coating states ofplating-layer-containing metal on cut faces under different lasercutting and machining conditions.

FIG. 8 shows photos of plasma generating states.

FIG. 9 shows enlarged photos of corrosion resistance evaluation resultsof cut faces.

FIG. 10 shows a diagram of exposure test evaluation results and photosof plasma generation.

FIG. 11 shows a diagram of exposure test evaluation results and photosof plasma generation.

FIG. 12 shows a diagram of exposure test evaluation results and photosof plasma generation.

FIG. 13 shows a diagram of exposure test evaluation results and photosof plasma generation.

FIG. 14 shows a diagram of exposure test evaluation results and photosof plasma generation.

FIG. 15 shows a diagram of exposure test evaluation results and photosof plasma generation.

FIG. 16 shows a diagram of exposure test evaluation results and photosof plasma generation.

FIG. 17 shows a diagram of exposure test evaluation results and photosof plasma generation.

FIG. 18 is an explanatory diagram showing relationships between plasmageneration and red rust occurrence based on exposure tests of cleancutting and easy cutting.

FIG. 19 shows photos of EDS analysis results and red rust occurrencesbased on different processing speeds.

FIG. 20 is a model showing a laser cutting state.

FIG. 21 is a model showing the definition of a melted plating width.

FIG. 22 is a model showing the definition of a plating metal coatingstate on a cut face.

FIG. 23 is a diagram explaining an inflow state of a plating metal layerat laser cutting.

FIG. 24 shows diagrams explaining a laser cutting configurationaccording to an embodiment of the present invention, in which (a) showsa relationship among a laser beam, a cutting gas nozzle, and a materialto be cut and (b) shows pressure distributions of a cutting gas and anauxiliary gas acting on the material to be cut.

FIG. 25 shows diagrams explaining a formation of a plating metal layeraccording to an embodiment of the present invention, in which (a) showsa laser cutting start state of the plating metal layer and (b) shows apost-state of the plating metal layer.

FIG. 26 shows diagrams explaining a conventional laser cuttingconfiguration, in which (a) shows a relationship among a laser beam, acutting gas nozzle, and a material to be cut and (b) shows pressuredistributions of a cutting gas acting on the material to be cut.

FIG. 27 shows diagrams explaining a conventional formation of a platingmetal layer, in which (a) shows a laser cutting start state of theplating metal layer and (b) shows a post-state of the plating metallayer.

FIG. 28 shows a laser cutting example according to an embodiment of thepresent invention.

FIG. 29 shows another laser cutting example according to an embodimentof the present invention.

FIG. 30 explains methods of measuring a coating layer, in which (a)shows a method of measuring a ratio (plating inflow length ratio) of acoating layer average length to a plate thickness and (b) shows a methodof measuring a coverage (coating ratio) of a coating layer on a cutface.

FIG. 31 explains a method of measuring the thickness of an oxidizedlayer or a nitrided layer, in which (a) shows a section of a measurementsample in which a test material is buried in resin and (b) shows asection and polished surface of the measurement sample after polishing.

FIG. 32 explains a rusting ratio on a cut end face.

FIG. 33 shows an external appearance of a cut end face according to anembodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

Hereunder, embodiments according to the present invention will beexplained with reference to the diagrams.

FIG. 1 is a configuration explanatory diagram schematically showing aconfiguration of a laser cutting and machining apparatus according to anembodiment of the present invention.

Referring to FIG. 1 , the laser cutting and machining apparatus 1according to an embodiment of the present invention has a work table 3supporting a plate-like work W and a laser machining head 5 thatirradiates the work W with a laser beam LB and cuts and machines thework W with the laser beam. The work table 3 is arranged to be movablein X- and Y-axis directions relative to the laser machining head 5. Torelatively move and position the work table 3 in the X- and Y-axisdirections, a positioning motor 7 such as a servomotor is arranged. Alsoarranged to move and position the laser machining head 5 relative to thework W in an approaching/distancing direction (Z-axis direction) is aZ-axis motor 9.

Further, the laser cutting and machining apparatus 1 has a laseroscillator 11 such as a CO₂ laser oscillator that oscillates a laserbeam in a far-infrared wavelength range (a laser beam of 3 μm or more inwavelength). The laser machining head 5 has optical devices 17 such as areflection mirror 13 that reflects the laser beam LB oscillated by thelaser oscillator 11 toward the work W and a condenser lens 15 thatcondenses the laser beam LB. The laser machining head 5 also has adetachable and replaceable nozzle 19 that jets an assist gas to a lasercutting and machining position of the work W.

As a configuration to jet an assist gas to a laser cutting and machiningposition, it is possible to provide the laser machining head 5 with aside nozzle from which the assist gas is jetted toward a laser machiningpart.

The laser cutting and machining apparatus 1 further has an assist gassupply device 21. The assist gas supply device 21 supplies a mixed gasof, for example, approx. 97% nitrogen gas and approx. 3% oxygen gas andincludes a nitrogen gas supply device 23, an oxygen gas supply source(air supply source) 25, and a mixer 27 to produce the mixed gas.Further, the assist gas supply device 21 has a pressure adjustment valve29 that adjusts a pressure of the assist gas to be supplied to the lasermachining head 5. If the oxygen gas supply source 25 of the assist gassupply device 21 is stopped and only the nitrogen gas supply device 23is operated, only a nitrogen gas will be supplied as the assist gas to apart to be processed.

A configuration of supplying the mixed gas of approx. 97% nitrogen gasand approx. 3% oxygen gas as an assist gas to a laser processing part isnot limited to the one mentioned above. Any other configuration ispossible. For example, as stipulated in Japanese Patent Publication No.3291125, it is possible to separate, by a separating device employing ahollow fiber membrane, nitrogen and oxygen from each other fromcompressed and supplied air. The laser cutting and machining employing,as an assist gas, the mixed gas of approx. 97% (96% or more) nitrogenand approx. 3% (4% or smaller) oxygen will simply be referred to as“easy cutting”.

The laser cutting and machining apparatus 1 also has a control device31. The control device 31 is a computer having a function of controllingthe moving and positioning of the laser machining head 5 relative to thework W, a function of controlling a laser output of the laser oscillator11, and a function of controlling a supplying pressure of the assist gasto the laser machining head 5.

With the above-mentioned configuration, the work W is set and positionedon the work table 3, and thereafter, the laser machining head 5 is movedand positioned in the X-, Y-, and Z-axis directions relative to the workW. The laser beam LB oscillated by the laser oscillator 11 is condensedthrough the condenser lens 15 to irradiate the work W. The assist gassupplied from the assist gas supply device 21 to the laser machininghead 5 is jetted from the nozzle 19 toward a laser machining part of thework W, which is thus cut and machined.

If the work W to be laser cut and machined is a plated steel plate,evaporated matter of a plating layer of the plated steel plate may entera processed area as shown in FIG. 21 of the Patent Literature 1 and maycause processing quality defects. For this, the Patent Literature 1stipulates that, as shown in FIG. 1 thereof, the surface of the platedsteel plate is irradiated with a laser beam to remove the plating layerin advance, and thereafter, the laser cutting and machining is carriedout along the same path.

According to this configuration, there is no plating evaporation duringthe laser cutting and machining, and therefore, the quality ofprocessing may improve. However, it requires the plating layer removingprocess and cutting process, i.e., two times of laser processing. Inaddition, a cut face of the plated steel plate is left in the laser cutand machined state, and therefore, there is a problem of necessitating arustproofing process.

Embodiments of the present invention are based on findings that, whenlaser cutting and machining a plated steel plate, melting and/orevaporating a plating layer on a top surface of the plated steel platecauses melted and/or evaporated plating-layer-containing metal to flowto a cut face and that the flowed plating-layer-containing metal cancoat the cut face.

According to an embodiment of the present invention, employed as anexample of the plated steel plate is a hot-dipped steel plate(hereinafter, simply referred to as a “plated steel plate”) that is asteel plate coated with a plating layer of 6% aluminum, 3% magnesium,and the remaining 91% zinc.

A laser cutting process generally carried out is oxygen cutting whichuses an oxygen gas as an assist gas. According to an EPMA (ElectronProbe Micro Analyzer) analysis, the oxygen cutting covers, as shown inFIG. 2 , a cut face with an oxide film.

Next, a laser cutting and machining method employing a nitrogen gas asan assist gas (hereinafter, simply referred to as “clean cutting”)demonstrates, depending on cutting conditions, a satisfactory lasercutting and machining result on a cut face CF of a base material B of aplated steel plate, as shown in enlarged photos of FIG. 3 (A). At arounda top end part of the cut face CF, a plating layer M on the top surfaceis removed and is very thin. The cut face CF has no oxide film and thelike and substantially shows only an original plate component (Fe) ofthe plated steel plate (refer to FIG. 2 ). A coating layer (platinglayer) on the cut face CF is very thin. Consequently, the clean cuttingis capable of, depending on proper cutting conditions, coating the cutface CF with melted plating-layer-containing metal of the top surfaceand sometimes causing no rust (red rust).

Next, the easy cutting produces, as shown in FIGS. 2 and 3 (C), a thinoxide film appears on a cut face. At an upper part of the cut face,components of a plating layer M such as zinc, aluminum, and magnesiumappear. Namely, at around the upper end part of the cut face CF, meltedplating layer partly flows to the cut face CF to produce white stripswith thick flows of the melted plating layer. Gaps among the strips showthin films of the melted plating layer.

Namely, it has been found that the clean cutting or easy cutting whichis a generally adopted laser cutting and machining method for a steelplate is able to make metal contained in the plating layer M flow towardand coat the cut face CF of the plated steel plate (work) W.

For this, processing conditions such as a laser cutting speed, acondenser lens focal position, an assist gas pressure, and a laser beampulse frequency are variously changed to test a plating layer coatingstate on a cut face. Test conditions are as mentioned below.

Laser cutting machine: Amada Co., Ltd. FOM2-3015RI

Material: Plated steel plate coated with plating of 6% aluminum, 3%magnesium, remaining 91% zinc, plate thickness t=2.3 mm, K35 (platingquantity 175 g/m² per face)

Cut sample shape: 130 mm×30 mm

Standard processing conditions

-   -   Nozzle diameter: D4.0 (4.0 mm)    -   Cutting speed: F1600 (1600 mm/min)    -   Assist gas type: EZ (an assist gas that is used for the        above-mentioned easy cutting and is a mixed gas of about 97%        nitrogen and 3% oxygen)    -   Assist gas pressure: 0.9 MPa    -   Nozzle gap: 0.3 mm (a gap between a nozzle and a top surface of        a plated steel plate)    -   Focal position: −4.5 mm (work top surface being 0, upper side        thereof being +, and lower side thereof being −)

Processing results obtained by changing these standard processingconditions are as mentioned below.

As is apparent from FIG. 4 , when adjusting the cutting speed within arange of 1120 mm/min to 3840 mm/min, the plating metal coating amount ona cut face (cut end face) gradually increases as the cutting speedincreases.

As is apparent from FIG. 5 , when adjusting the condenser lens focalposition within a range of −6.5 mm to +0.5 mm, the plating metal coatingamount on a cut face gradually increases as the focal position isgradually adjusted to the + side.

As shown in FIG. 6 , when adjusting the assist gas pressure within arange of 0.5 MPa to 0.9 MPa, the plating metal coating amount on a cutend face gradually increases as the assist gas pressure is decreased.

As shown in FIG. 7 , when adjusting the laser beam pulse frequencywithin a range of 800 Hz to CW (continuous), the plating metal coatingamount on a cut end face shows no significant change.

The results shown in FIGS. 4 to 7 tell that, in the easy cutting, theplating-layer-containing metal coating amount on a laser cut face of aplated steel plate increases as the cutting speed becomes higher (forexample, 3840 mm/min). As the focal position moves to the + side (forexample, +0.5 mm), the plating-layer-containing metal coating amountincreases. However, greatly moving the focal position to the + sideresults in lowering an energy density on the top surface of a platedsteel plate, and therefore, it is preferable in the laser cutting andmachining, to set the same on the − side. As the assist gas pressuredecreases (for example, 0.5 MPa), the plating-layer-containing metalcoating amount increases. Adjusting the laser beam between pulsatory andcontinuous causes no significant change in the plating metal coatingamount.

As will already be understood, when conducting the easy cutting (EZ) tocut and machine a plated steel plate with a laser, varying laser cuttingand machining conditions such as the cutting speed, condenser lens focalposition, and assist gas pressure results in changing aplating-layer-containing metal coating amount on a laser cut face of theplated steel plate. Varying the laser cutting and machining conditionsmay include changing a gap between the nozzle 19 of the laser machininghead and the top surface of the work W, i.e., a nozzle gap.

Namely, it is understood that the plating-layer-containing metal coatingamount on a laser cut face of a plated steel plate is dependent on thelaser cutting and machining conditions on the plated steel plate. Inother words, the easy cutting of a plated steel plate, if carried outunder proper laser cutting conditions, is able to properly coat a lasercut face with plating-layer-containing metal.

It has been found that the easy cutting is able to coat a cut face of aplated steel plate with plating-layer-containing metal.

Next, in order to find out proper cutting conditions for the cleancutting, various cutting conditions are applied to laser cut and machineplated steel plates, and in order to observe red rust occurring stateson laser cut faces, an exposure test is carried out. The exposure testholds upward the cut face of a laser cut-and-machined plated steel plateand leaves the same as it is in the open air for one month.

When the clean cutting is carried out to cut a plated steel plate into alaser cut-and-machined product, there are, as shown in FIG. 8 , a caseto cause plasma on a top surface of a laser cut-and-machined positionand a case to cause no plasma. In the case of causing plasma, a weakplasma generation and a strong (not weak) plasma generation are visuallydistinguishable from each other. The case of no plasma generation isclassified as “Nil”, weak plasma generation as “p”, and strong plasmageneration as “P”. If the cutting conditions are improper to accomplishthe laser cutting and machining, it is classified as “NG”.

In the one-month exposure test, no red rust occurrence is representedwith “O” and red rust occurrence with “X” as shown in FIG. 9 . Theexposure test is a result of one month passage in the open air.Accordingly, some of the laser cut-and-machined products evaluated as“X” are usable depending on environments of use.

Next, the clean cutting with a nitrogen gas as an assist gas is carriedout on plated steel plates having plate thicknesses t=2.3 mm, t=3.2 mm,t=4.5 mm, and t=6.0 mm and the exposure test is carried out on them.Results of the tests are as shown in FIGS. 10 to 17 . In FIGS. 10 to 17, K14, K27, and K35 each are a plating quantity indicator. They are K14(per-face plating quantity 70 g/m²), K27 (per-face plating quantity 145g/m²), and K35 (per-face plating quantity 175 g/m²).

In FIGS. 10 to 17 , S indicates a single nozzle and D a double nozzle(dual nozzle). The double nozzle is already known as stipulated in, forexample, Japanese Unexamined Patent Application Publication No.H11-90672. S2.0, D4.0, and D7.0 each indicate a nozzle diameter (mm).Namely, S2.0=2.0 mm, D4.0=4.0 mm, and D7.0=7.0 mm. Corresponding to thenozzle diameters, nozzle gaps are set as 0.3 mm for S2.0, 0.5 mm forD4.0, and 1.0 mm for D7.0. Namely, as the nozzle diameter increases,spatters and the like produced at a laser processing position easilyenter into the nozzles, and therefore, the nozzle gap is set to becomelarger as the nozzle diameter increases.

Laser processing parameters other than those particularly mentioned arethe same as those of the above-mentioned standard processing conditions.

Referring to FIG. 10 , the focal position is −0.5 mm (the focal positionis shown in each diagram), the plate thickness is t=2.3 mm, and theplating quantity is K14. When the nozzle diameter is S2.0, no plasmaoccurs at 1000 mm/min under the assist gas pressures of 0.9 MPa, 0.7MPa, and 0.5 MPa. The exposure test results are each “X” to indicate anentire red rust occurrence. As shown in FIGS. 11, 12, 13, 14, and 15 ,at the cutting speed of 1000 mm/min, the nozzle diameter of S2.0produces no plasma without regard to the assist gas pressures. Theexposure test results are “X” to indicate an undesirable rust proofingeffect on each cut face.

Accordingly, it is difficult for the laser cutting and machining of aplated steel plate with the nozzle diameter of S2.0 at the cutting speedof 1000 mm/min to make melted and/or evaporated plating-layer-containingmetal flow toward and coat a cut face.

In FIGS. 10, 11, and 12 , the cases with the nozzle diameter D4.0 willbe examined. In FIGS. 10 and 11 , there is no plasma generation and theexposure test results are “X”. In FIG. 11 , however, the case with theassist gas pressure 0.7 MPa improves to “O”. The cases with the nozzlediameter D7.0 show weak plasma generation. The exposure test results are“X” in FIG. 10 , “O” and “X” in FIG. 11 , and “X” in FIG. 12 .

In FIGS. 10 to 17 , among the exposure test results of “O” and “X”, thecases with the plasma generation (P) are almost “O”. Accordingly, it ispreferable to generate plasma when laser cutting and machining a platedsteel plate by the clean cutting, in order to make melted and/orevaporated plating-layer-containing metal flow to and coat a cut face.

As is apparent from FIG. 11 , the cases with the plating quantity K27and nozzle diameter D4.0 include one with no plasma generation and “O”in the exposure test result. In FIG. 10 , the cases with the nozzlediameter D7.0 and assist gas pressure 0.9 MPa include those with slightplasma generation and “X” in the exposure test results.

In FIGS. 10, 11, and 12 , the cases with the cutting speed in a range of3000 mm/min to 5000 mm/min each generate plasma and the plasmageneration becomes stronger as the cutting speed increases. In FIG. 10 ,all of the cases except those with the nozzle diameter S2.0 and theassist gas pressures 0.9 MPa and 0.7 MPa (3000 mm/min) are “O” in theexposure test results. In FIG. 10 , the cases with the nozzle diameterS2.0 mm, assist gas pressure 0.7 MPa, and 4000 mm/min to 5000 mm/min are“O”.

Therefore, the exposure test result of “O” is obtainable for the platedsteel plate thickness t=2.3 mm, plating quantity K14, and nozzlediameter S2.0 if the assist gas pressure is 0.7 MPa and the cuttingspeed in the range of 4000 mm/min to 5000 mm/min. For the assist gaspressure 0.5 MPa, the range of 3000 mm/min to 5000 mm/min is preferable.For the nozzle diameter of D4.0 or D7.0, it is preferable to set thecutting speed in the range of 3000 mm/min to 5000 mm/min without regardto the assist gas pressure of 0.9 MPa, 0.7 MPa, or 0.5 MPa.

As shown in FIG. 11 , increasing (thickening) the plating quantity toK27 on the same plated steel plate thickness (t=2.3 mm) with the nozzlediameter D4.0, assist gas pressure 0.7 MPa, and cutting speed 1000mm/min provides the evaluation “O” even without plasma generation.Accordingly, properly harmonizing the conditions of the plated steelplate thickness (t=2.3 mm), plating quantity K27, nozzle diameter D4.0,assist gas pressure 0.7 MPa, and cutting speed 1000 mm/min is able toprovide the evaluation “O” even without plasma generation. In otherwords, properly setting the above-mentioned conditions results in makingplating-layer-containing metal melted and/or evaporated during the lasercutting and machining flow to and coat a cut face even if no plasma isgenerated.

Referring to FIG. 12 , only the plating quantity is changed to K35. Atthe cutting speed 1000 mm/min, the case with the nozzle diameter D4.0and assist gas pressure 0.7 MPa and the cases with the nozzle diameterD7.0 and assist gas pressures 0.9 MPa and 0.7 MPa show slight plasmageneration but are evaluated as “X”.

Generally, if plasma is generated in the laser cutting and machining ofa metal plate, the plasma has a characteristic to absorb a laser beamand irradiation of the laser beam promotes continuous plasma generation.The plasma is known to worsen a cut face roughness. However, there is aplasma cutting method such as a stainless steel non-oxidation cuttingmethod that uses plasma heat. In this case, processing conditions areset to promote the growth of generated plasma.

In this case, (i) the assist gas is set to a low pressure, (ii) thenozzle gap between the nozzle and a work is slightly extended than anormal case, to form a space for growing plasma, (iii) the focalposition is moved toward the (+) direction than a normal focal position,where (+) is an upward direction above a work surface and (−) is adownward direction below the work surface, and (iv) the cutting speed isincreased to reduce a laser beam heat gain of the work. These conditions(i) to (iv) promote plasma generation when laser cutting and machining ametal plate.

Considering these conditions (i) to (iv), FIG. 10 will be examined. Atthe plating quantity K14 and in a cutting speed range of 1000 mm/min to2000 mm/min, the nozzle diameter D4.0 than S2.0 and the nozzle diameterD7.0 than D4.0 generate more plasma. As the cutting speed graduallyincreases from 1000 mm/min to 5000 mm/min, the plasma generation becomesstronger. As the plasma generation becomes stronger, the exposure testresults involve more “O”. The same tendencies are also observed in FIGS.11 and 12 .

Accordingly, it is better at the laser cutting and machining of a platedsteel plate to generate plasma to make melted and/or evaporatedplating-layer-containing metal of the top surface of the plated steelplate flow to and coat a cut face.

FIGS. 13, 14, and 15 are exposure test results of the clean cuttingunder the conditions of a plated steel plate thickness t=3.2 mm andplating quantities K14, K27, and K35. In FIGS. 13 to 15 , “NG” indicatesa cutting impossibility, i.e., improper cutting conditions. As isapparent from the results shown in FIGS. 13 to 15 , lower assist gaspressures and higher cutting speeds tend to easily generate plasma.

FIGS. 16 and 17 are exposure test results of the clean cutting withplate thicknesses t=4.5 mm and t=6.0 mm. These cases also show thatlowering the assist gas pressure, i.e., enlarging the nozzle diameterand increasing the cutting speed tend to strengthen plasma generation.As the plasma generation becomes stronger, the exposure test resultstend to show “O”. In FIGS. 16 and 17 , “D” indicates that a drossdeposition amount is large.

The exposure test results shown in FIGS. 10 to 17 are stored in acutting conditions data table 33 provided for the control device 31.Namely, the cutting conditions data table 33 stores, for each platedsteel plate thickness, processing conditions data such as nozzlediameters adopted for plating quantities, a nozzle gap for each nozzlediameter, a focal position, and cutting speeds. Further, the cuttingconditions data table 33 stores plasma generation data at the time oflaser cutting and machining of plated steel plates and exposure testresults. The control device 31 also includes a cutting conditions datatable that stores cutting conditions data for the easy cutting.

When various processing conditions are inputted through an input means35 connected to the control device 31, the laser cutting and machiningthat provides the same results as those shown in FIGS. 10 to 17 iscarried out. For example, if the conditions for the plate thicknesst=2.3 mm shown in FIG. 10 such as the plating quantity K14, nozzlediameter D4.0, assist gas pressure 0.7 MPa, and cutting speed 5000mm/min are inputted through the input means 35 into the control device31, plasma will be generated and the laser cutting and machining will becarried out accordingly. Then, the one-month exposure test to be carriedout thereafter will provide the evaluation result “O”.

The exposure test result may vary depending on environmental andmeteorological conditions such as seaside conditions.

To laser cut a plated steel plate work and make melted and/or evaporatedplating-layer-containing metal flow toward and coat a cut face, apreferable melting range of plating is 0.27 mm to 0.5 mm from a cut endface of the work, although it is dependent on a work plate thickness, aplating quantity, and laser cutting conditions.

If the plating layer melting and/or evaporating range is equal to orgreater than 0.5 mm, the laser cutting speed will slow and a heat gainwill be large. In this case, it is understood that a melted and/orevaporated plating metal amount increases to increase an inflow to alaser cut face. It is understood, however, that the slow laser cuttingspeed extends a laser beam irradiation time, i.e., a heating time tokeep the melted and/or evaporated plating-layer-containing metal at hightemperatures for a longer time and extend an assist gas acting time,thereby easily blowing off the melted and/or evaporatedplating-layer-containing metal before the same solidifies on the cutface, thus reducing a coating amount of the melted and/or evaporatedplating-layer-containing metal on the cut face (refer to, for example,D4.0 and D7.0 of FIG. 12 ).

If the melting and/or evaporating range of a plating layer is as smallas 0.27 mm, the laser cutting speed will be high and the heat gain willbe small. In this case, it is understood that the melted and/orevaporated plating metal amount is small to decrease an inflow to alaser cut face.

Accordingly, it is preferable that the plating layer melting and/orevaporating range is 0.27 mm to 0.5 mm from a cut face. Within thisrange, the laser beam irradiation time and assist gas acting time areappropriate to reduce the melted and/or evaporated plating metal amountblown off by the assist gas. This may result in easily coating a cutface with the melted and/or evaporated plating metal and solidifying thesame at there, thereby increasing the plating-layer-containing metalcoating amount (refer to, for example, D4.0 and D7.0 in FIG. 12 ).

As is already understood, the laser cutting and machining of a platedsteel plate according to the clean cutting employing a nitrogen gas asan assist gas or the easy cutting employing a mixed gas of about 97%nitrogen and about 3% oxygen as an assist gas is able to coat a cut facewith plating-layer-containing metal of a top surface of the plated steelplate. It has been found that plasma generation during the laser cuttingand machining effectively carries out the coating.

The clean cutting and easy cutting are carried out on plated steelplates having a plate thickness t=2.3 mm and the plasma generationobservation and one-month exposure test are conducted. Results are shownin FIG. 18 .

The results shown in FIG. 18 suggest that each of the clean cutting andeasy cutting is able to, if plasma is generated during the lasercutting, effectively coat a cut face with plating-layer-containing metaland prevent the occurrence of red rust.

As the processing speed increases, the plating-layer-containing metalmore effectively coats a cut face and prevents the occurrence of redrust. At processing speeds 2200 mm/min and 5000 mm/min, plated steelplates having a plate thickness t=2.3 mm and plating quantity K14 arelaser cut. Observation results of cut faces thereof are shown in FIG. 19.

As is apparent from FIG. 19 , the processing speed 2200 mm/min causesred rust. At the processing speed 5000 mm/min, however,plating-layer-containing metal components are detected on an entire cutface and no red rust is observed. These results agree with the resultsshown in FIG. 18 .

From EDS (Energy Dispersive X-ray Spectrometry) analysis results andexposure test results (after four weeks) of the laser cut faces shown inFIG. 19 , the followings are understood. According to the clean cuttingof FIG. 2 , a very small amount of plating-layer-containing metal isdetected on a laser cut face. The EDS analysis of the laser cut face cutat the processing speed 2200 mm/min that is nearly equal to the cleancutting condition of FIG. 2 shows that plating-layer-containing metalcomponents such as Zn, Al, and Mg are less than photographablequantities and are substantially not coating the laser cut face. Whenthe cutting conditions are changed to proper ones, as is apparent in theEDS analysis result photographs at the processing speed 5000 mm/min inFIG. 19 , the plating-layer-containing metal is detected allover thelaser cut face to suggest that the laser cut face is entirely coatedwith the plating-layer-containing metal. Namely, the cut face cut undera normal condition (processing speed: 2200 mm/min) shows about 90% iron(Fe weight percent: 89.16) and no detectable plating components (Zn, Al,and Mg each being equal to or less than 1.45 weight percent). Due tothis, red rust easily occurs. On the other hand, the processingcondition of this time (processing speed: 5000 mm/min) indicates thatthe iron on a cut face drastically decreases to about 30% (Fe weightpercent: 32.48), Zn greatly increases to 43.57 weight percent, Al and Mgincrease several times, and the plating components entirely cover thecut face. It is understood, therefore, that the occurrence of red rustis suppressed by the plating components that flow from a top surface ofthe plated steel plate during the laser cutting and machining and coverthe surface of the cut face.

As is understood from the embodiment explained above, if the lasercutting and machining of a plated steel plate is carried out underproper conditions based on the thickness and plating quantity of theplated steel plate, melted and/or evaporated plating-layer-containingmetal of a top surface of the plated steel plate flows to a cut faceduring the laser cutting and machining and easily coats the cut face.Accordingly, the thickness of a plating layer around a top edge of thecut face of the plated steel plate is thinner than the thickness of aplating layer at a position away from the cut face, i.e., a positionthat is thermally not affected so that the playing layer may not melt,evaporate, or flow during the laser cutting and machining.

The plated steel plate examples mentioned in the above explanationcontain 6% aluminum, 3% magnesium, and the remaining 91% zinc. Platedsteel plates are not limited to them and other kinds of plated steelplates are applicable.

Next, a second embodiment will be explained.

This embodiment employs, as a raw material, a surface-treated steelplate coated with plating metal on the surface of a steel plate, cutsthe raw material by a gas cutting method or a thermal cutting methodusing light energy or electric energy, and provides surface-treatedsteel plates used for automobiles, house appliances, power distributionfacilities, and communication facilities.

Conventionally, parts (surface-treated steel plates) used forautomobiles, house appliances, power distribution facilities,communication facilities, and the like are made by cutting a cold-rolledsteel plate into required sizes and carrying out hot-dip plating(post-plating) on them. In recent years, to improve corrosion resistanceand durability and reduce processes to save costs, widely used are partsthat employ a surface-treated steel plate as a raw material to omit thepost-plating. The surface-treated steel plate is frequently a platedsteel plate that is a steel plate whose surface is coated with metalsuch as Zn, Zn alloy, Al, Al alloy, and Cu. A most general method ofcutting such a surface-treated steel plate is to employ a press machineor punching with a mold conforming to the shape of a part. To suppressthe cost of the mold, a thermal cutting method is increasingly used. Thethermal cutting method is typically gas cutting, laser cutting using alaser beam that is light energy, or plasma cutting using plasma that iselectric energy. In the case of using a cut part as it is, the cuttingmethod employing light energy or electric energy is frequently adoptedbecause it causes little damage on the plating metal of asurface-treated steel plate and provides a cut face having a goodappearance.

A related patent literature is Japanese Unexamined Patent ApplicationPublication No. 2001-353588.

Usually, a process of thermally cutting a surface-treated steel plateinto an optional shape with the use of light energy or electric energyforms a thickness-direction cut face where top and bottom plating layersare removed to expose a steel base. As a result, the cut face has a lowrustproofing ability, and depending on a placed environment, quicklycauses red rust. The red-rusted part has a poor appearance and corrodesto reduce volume to raise a problem of unsatisfying a required strength.The problem is serious in a thicker item because the rusted appearanceeasily catches attention and because the rust affects strength. To copewith this, a prior art applies, after the thermal cutting, a repairingpaint, which has components similar to the plating metal components, tothe cut face. This raises another problem of adding the costs of paintand painting process, thereby increasing the cost of the product.

A conventional thermal cutting method for a plated steel plate isproposed in the patent literature mentioned above. This method employs,as an assist gas, a mixed gas of 2 to 20% oxygen and nitrogen to improvea cutting efficiency. According to an embodiment thereof, a zinc-platedsteel plate of 3 mm thick is cut under an assist gas pressure of 12 bar(1.2 MPa) and a cutting speed of 1.8 m/min. However, as will beexplained with reference to the embodiment, a cut face of asurface-treated steel plate cut according to the conventional method hasinsufficient plating components, and therefore, achieves a poorrustproofing ability.

The second embodiment has been devised to resolve the above-mentionedproblems and an object thereof is to cut, as a raw material, asurface-treated steel plate coated with plating metal by a thermalcutting method utilizing light energy or electric energy such thatplating-layer-containing metal covers a cut face to secure arustproofing ability for the cut face.

The second embodiment employs a surface-treated steel plate as a rawmaterial to form a thermally cut part and secures a rustproofing abilityon a cut face without repair painting by making plating-layer-containingmetal melted by thermal cutting flow to the cut face.

Adopted as the surface-treated steel plate is a plated steel platecoated with Zn, Zn alloy, or the like.

Namely, the surface-treated steel plate having a plating layer is asteel plate whose surface is coated with plating metal.Plating-layer-containing metal on the surface of the steel plate melts,flows to a cut face, and solidifies to cover the cut face, therebyproviding a thermally cut product having an excellent cut-face corrosionresistance.

In the above, the raw material may be a hot-dip Zn-based plated steelplate having a plating composition of 0.1 to 22.0% Al in weight percent.Also, it may be a hot-dip Zn-based plated steel plate containing one ormore selected from a group of 0.1 to 10.0% Mg, 0.10% or lower Ti, 0.05%or lower B, and 2% or lower Si. Also, the raw material may adopt alloyedZn plating.

The second embodiment is able to provide a cut product with arustproofing ability of at least one month or more, and due to nonecessity of repair painting on a cut face after cutting, involves nopaint cost nor painting process.

The cut product according to the second embodiment is characterized asthe plating-layer-containing metal of a raw material, i.e., in which asurface-treated steel plate is present on a thermally cut face. Namely,the plating-layer-containing metal of a part of the steel plate surfacecovers the cut face. The surface-treated steel plate is not limited to aparticular kind. In consideration of corrosion resistance and damages oncoating components during thermal cutting, it is preferable to use aplated steel plate coated with Zn or Zn alloy. A source plate of thesurface-treated steel plate is not particularly limited. It may be ahot-rolled steel plate or a cold-rolled steel plate. A steel type may beextra-low carbon steel or low carbon steel. A plate thickness andplating quantity are also not particularly limited. They may bedetermined in consideration of corrosion resistance and strengthrequired for thermally cut parts. For example, if the corrosionresistance is important, the quantity of plating will be increased toincrease the quantity of plating-layer-containing metal flowing to a cutface at the time of thermal cutting.

A thermal cutting method for manufacturing a thermally cut part employslight energy or electric energy in consideration of appearance of a cutface. The light energy cutting method includes CO₂ laser cutting, YAGlaser cutting, and fiber laser cutting. The electric energy cuttingmethod includes plasma cutting and arc cutting. The second embodimentproduces a thermally cut part by employing the above-mentioned cuttingmethod to melt plating-layer-containing metal on the surface of a steelplate around a cut area at the time of cutting and by using an assistgas to flow the melted plating-layer-containing metal into a cut face.Flowing easiness of the melted plating-layer-containing metal to the cutface varies depending on cutting conditions such as a heat gain and anassist gas pressure at the time of cutting. If the heat gain is toohigh, the melted plating-layer-containing metal will evaporate beforeflowing to the cut face. If the heat gain is too low, theplating-layer-containing metal will insufficiently melt, or the cuttingof the steel plate will not be completed. If the assist gas pressure istoo high, the plating-layer-containing metal flowing to the cut facewill excessively be blown off.

Conditions of a surface-treated steel plate serving as a raw materialalso affect the rustproofing ability of a cut face. As the quantity ofplating increases, a ratio of melting plating-layer-containing metal toa given heat gain at the time of thermal cutting increases. As a platethickness becomes thinner, the area of a cut face to be coated withplating-layer-containing metal becomes smaller. Due to these reasons,the raw material conditions that affect the flowing ofplating-layer-containing metal to a cut face and improve a rustproofingability are a larger plating quantity and a thinner plate thickness.

The inventers of the present invention have found that combinations ofthese cutting conditions and raw material conditions realize conditionsto easily pass plating-layer-containing metal to a cut face at the timeof thermal cutting. Based on such conditions, the inventors havecompleted cut parts that have an improved cut face rustproofingcharacteristic.

Embodiment 1

Hereunder, an embodiment of the present invention will be explained.

Raw materials employed are, as shown in Table 1, a Zn-6% Al-3% Mg-platedsteel plate, a Zn-plated steel plate, and an Al-plated steel plate. Afacility employed is a most widely used CO₂ laser to cut the steelplates under combinations of various conditions shown in Table 2.

TABLE 1 Plating type Zn-6% Al-3% Mg, Zn, Al Plating quantity per face 60to 175 g/m² Plate thickness 2.3 to 3.2 mm

TABLE 2 Cutting speed 1.4 to 3.0 m/min Pulse output 4 to 5 kW Nozzlediameter ϕ2 to 4 mm Nozzle gap 0.3 mm Assist gas type N₂, N₂ + 3 Vol %O₂ Assist gas pressure 0.5 to 1.2 MPa Beam diameter ϕ0.2 to 0.3 mm

FIG. 20 shows a raw material to be cut. A top surface side to receive alaser beam in a dashed-line area causes partial melting of a platinglayer of the raw material, and after cutting, the melted part of theplating layer becomes thinner than an original thickness of the platinglayer due to evaporation or flowing to a cut face . In connection withthis, a melted plating width L shown in FIG. 21 , i.e., the width of anarea in which the plating layer is thinned is examined on the topsurface of the raw material. As shown in FIG. 22 , a coating state ofplating-layer-containing metal on the cut face is observed to examine aratio of a plating-layer-containing metal coating area to a cut facearea and a ratio of a maximum inflow distance of theplating-layer-containing metal (a maximum inflow size of theplating-layer-containing metal from the top surface of the raw material,i.e., the surface-treated steel plate along the cut face) to a platethickness. Each cut sample is subjected to an outdoor exposure test tomeasure the number of days until visible rust occurs and see if thenumber of days exceeds one month.

Results thereof are shown in Table 3. Cut products having a meltedplating width L of 0.27 mm to 0.5 mm and a meltedplating-layer-containing metal coating area ratio of 10% or greater or aplating-layer-containing metal maximum flow distance to plate thicknessratio of 30% or greater each demonstrate a rustproofing ability of onemonth or longer.

As a comparison, a cut face cut according to conditions (No. 19 of Table3) stipulated in the patent literature (Japanese Unexamined PatentApplication Publication No. 2001-353588), i.e., a cutting speed of 1.8m/min and an assist gas pressure of 12 bar (1.2 MPa) is examined. Themelted plating width thereof exceeds the range of the second embodimentand no one-month rustproofing ability is observed.

TABLE 3 Plating Plate Cutting Pulse Nozzle Nozzle Plating quantitythickness speed output diameter gap No. type per face (mm) (m/min) (kW)(mm) (mm) Assist gas (O₂: 3%) 1 a 70 2.3 3.0 4 4.0 0.3 N₂ 2 a 70 2.3 3.04 4.0 0.3 N₂ + O₂ 3 a 70 2.3 1.6 4 2.0 0.3 N₂ 4 a 70 2.3 1.6 4 2.0 0.3N₂ + O₂ 5 b 60 3.2 2.0 5 4.0 0.3 N₂ 6 b 60 3.2 1.4 5 2.0 0.3 N₂ 7 c 603.2 1.4 5 2.0 0.3 N₂ + O₂ 8 a 70 3.2 1.4 5 2.0 0.3 N₂ 9 a 70 3.2 1.4 52.0 0.3 N₂ + O₂ 10 a 145 3.2 2.0 5 4.0 0.3 N₂ 11 a 145 3.2 2.0 5 4.0 0.3N₂ + O₂ 12 a 145 3.2 1.4 5 2.0 0.3 N₂ 13 a 145 3.2 1.4 5 2.0 0.3 N₂ + O₂14 a 175 3.2 2.0 5 4.0 0.3 N₂ 15 a 175 3.2 2.0 5 4.0 0.3 N₂ + O₂ 16 a175 3.2 1.4 5 2.0 0.3 N₂ 17 a 175 3.2 1.4 5 2.0 0.3 N₂ + O₂ 18 a 175 3.21.8 5 4.0 0.3 N₂ + O₂ 19 b 60 3.2 2.0 5 2.0 0.3 N₂ 20 a 70 2.3 2.0 4 2.00.3 N₂ + O₂ Max ratio Assist Focal Melted of plating gas Beam posi-plating Plating inflow length One-month pressure diameter tion widthcoverage to plate corrosion No. (MPa) (mm) (mm) L(mm) (%) thickness (%)resistance Remarks 1 0.5 0.2 0.5 0.32 69 64 ∘ Invention 2 0.5 0.2 0.50.36 73 48 ∘ Invention 3 0.9 0.2 0.5 0.53 27 23 x Prior art 4 0.9 0.20.5 0.57 16 17 x Prior art 5 0.6 0.3 0.5 0.27 77 40 ∘ Invention 6 0.90.3 0.5 0.63 9 31 x Prior art 7 0.9 0.3 0.5 0.72 12 26 x Prior art 8 0.90.3 0.5 0.66 28 29 x Prior art 9 0.9 0.3 0.5 0.70 9 21 x Prior art 100.6 0.3 0.5 0.37 73 68 ∘ Invention 11 0.6 0.3 0.5 0.36 69 52 ∘ Invention12 0.9 0.3 0.5 0.61 13 26 x Prior art 13 0.9 0.3 0.5 0.63 28 29 x Priorart 14 0.6 0.3 0.5 0.39 57 38 ∘ Invention 15 0.6 0.3 0.5 0.40 61 45 ∘Invention 16 0.9 0.3 0.5 0.68 22 38 x Prior art 17 0.9 0.3 0.5 0.64 3834 x Prior art 18 1.2 0.3 0.5 0.72 18 20 x Prior art 19 0.9 0.3 0.5 0.459 31 ∘ Invention 20 0.9 0.2 0.5 0.50 16 17 ∘ Invention Plating type; aZn—6% Al—3% Mg, b Zn, c Al

When a surface-treated steel plate that is a steel plate whose surfaceis coated with plating metal is cut and machined, there is provided asurface-treated steel plate having a cut face coated with metalcontained in the plating layer of the surface of the steel plate. Inthis case, it is preferable that an area on the cut face coated with theplating-layer-containing metal of the steel plate surface is 10% or moreof an area of the cut face and the coat by the plating-layer-containingmetal of the steel plate surface extends from the top or bottom surfaceof the surface-treated steel plate for 30% or more of a plate thickness.It is also preferable that the width of a thin part of the plating layerperpendicularly to the cut face is 0.27 mm to 0.5 mm.

In FIGS. 20 to 22 , a reference numeral 101 indicates the raw material,102 the laser beam, 103 the cut face, 104 the plating metal, 105 the cutface, 106 the maximum inflow distance of the plating-layer-containingmetal, and 107 an advancing direction of the laser beam.

Next, a third embodiment will be explained.

The third embodiment relates to a member having an excellent corrosionresistance. A surface-treated steel plate coated with plating metal isadopted as a raw material and is cut by laser into the member. It alsorelates to a laser cutting method for such a member.

Conventionally, members used for automobiles, house appliances, powerdistribution facilities, communication facilities, and the like are madeby cutting a cold-rolled steel plate into given dimensions andthereafter by entirely plating (post-plating) the cut steel plate. Inrecent years, to improve the corrosion resistance and durability ofmembers and save costs by reducing manufacturing processes, asurface-treated steel plate is widely used as a raw material to omit thepost-plating when making such members. The surface-treated steel plateis mainly a plated steel plate that is made by coating the surface of asteel plate with metal such as Zn, Zn alloy, Al, Al alloy, and Cu. Amethod of cutting such a surface-treated steel plate into a membergenerally employs a press machine or punching that uses a moldconforming to the shape of the member. To suppress the cost of such amold, a thermal cutting method is increasingly adopted. The thermalcutting method is gas cutting that burns a gas, laser cutting that usesa laser beam, or plasma cutting that uses heat plasma. The laser cuttingis frequently used because it causes little damage on the plating metalof a surface-treated steel plate and provides a cut face with a goodappearance.

A laser cutting method for a plated steel plate is proposed in, forexample, a patent literature (Japanese Unexamined Patent Application No.2001-353588). This method employs, as an assist gas, a nitrogen-oxygenmixed gas containing 2 to 20% oxygen, to improve a cutting efficiency.

When a surface-treated steel plate is cut into an optional shape bylaser cutting, a cut face along a plate thickness direction generallyexposes a steel base. Such a cut face has a low corrosion resistance, toquickly produce red rust depending on an environment of use and exhibita bad appearance. A product with red rust reduces its volume due tocorrosion to lack a required mechanical strength.

Since red rust on a thick product is apparently conspicuous and since athick product is required to have a practical mechanical strength, theabove-mentioned problems are serious on thick products. To cope withthis, a related art applies, after laser cutting a product, a repairingpaint having a function similar to that of plating metal to a cut faceof the product, in order to secure a corrosion resistance for theproduct. This countermeasure needs a paint and painting work to increasethe cost of the product.

The third embodiment has been devised to solve such problems and anobject thereof is to provide a member that uses a surface-treated steelplate coated with plating metal as a raw material and secures acorrosion resistance for a laser cut face. Another object is to providea laser cutting method to manufacture such a member.

The inventors of the present invention have made diligent studies toachieve the objects and found a phenomenon that, when laser cutting aplated steel plate, a plating metal layer on the surface of the steelplate melts due to laser radiation heat and flows toward a cut face.This finding has led to the completion of the embodiment. What isprovided by the embodiment will be explained in more detail.

(1) The third embodiment is a laser cutting method for a surface-treatedsteel plate having a plating metal layer on the surface of a steelplate, the method carrying out laser cutting with the use of a cuttinggas such as an oxygen gas, a nitrogen gas, or a mixed gas thereof toform a cut face and impinging an auxiliary gas to melted plating metallayer to make the melted plating metal layer flow to the cut face.

(2) The third embodiment is the laser cutting method as mentioned in(1), which arranges a plurality of nozzles for jetting the auxiliary gasaround a nozzle for the cutting gas and carries out the laser cutting.

(3) The third embodiment is the laser cutting method as mentioned in(1), which arranges a ring-shaped nozzle for jetting the auxiliary gasaround a nozzle for the cutting gas and carries out the laser cutting.

According to the third embodiment, the cut face is partly covered withthe plating metal layer so that a sacrificial anode effect may securecorrosion resistance allover the cut face. The cut face after the lasercutting needs no repair painting unlike the related art, therebyreducing manufacturing costs.

The laser cutting process is carried out as shown in FIG. 23 . To thesurface of a surface-treated steel plate 200, a laser machining head 5emits a laser beam LB, which is moved to melt and cut thesurface-treated steel plate 200 into a predetermined shape. The surfaceof the surface-treated steel plate 200 is coated with a plating metallayer 210 which evaporates around a cut face 220 (cut part) heated bythe emitted laser beam LB. At the time of laser cutting, the platingmetal layer in a region around the cut part is also heated by heatconduction of the laser beam. The plating metal (for example, Zn-basedand Al-based) has low melting and evaporating points, and therefore, aplating metal layer 230 in the region melts and partly evaporates. Themelted plating metal layer 230 has fluidity, and therefore, flows towardand onto the cut face, spreads over the cut face, and cools down tosolidify, thereby forming a coating layer 250 containing the platingmetal. The formation of the coating layer 250 containing the platingmetal secures corrosion resistance on the cut face like thesurface-treated steel plate, thereby producing a member having anexcellent end-face corrosion resistance. Without repair painting on thecut face after the laser cutting, a proper corrosion resistance can besecured. As shown in FIG. 23 , the coating layer 250 includes a partthat continuously spreads from the plating metal layer 210.

Laser cutting is usually carried out in such a way as to emit a laserbeam from a front end of an irradiation nozzle toward a cut material,and at the same time, jet a cutting gas (assist gas) from around thelaser beam toward the cut material. The cutting gas is used to expel anevaporated or melted material from a cut part. The inventors of thepresent invention have found that jetting an auxiliary gas toward aperipheral area of the cut part during the laser cutting promotes a flowof melted plating metal to a cut face due to a flow of the auxiliarygas. This embodiment arranges an auxiliary gas nozzle around a cuttinggas nozzle, to jet an auxiliary gas to the peripheral area of a cut partduring laser cutting.

(Surface-Treated Steel Plate)

A surface-treated steel plate to be used is not particularly limited. Itmay be a plated steel plate plated with Zn-based, Zn—Al-based,Zn—Al—Mg-based, Zn—Al—Mg—Si-based metal or an alloy thereof. A steelplate plated with a Zn—Al—Mg-based alloy is preferable. A base materialof the surface-treated steel plate may be a hot-rolled steel plate, acold-rolled steel plate, an extra-low carbon steel plate, or a lowcarbon steel plate.

The thickness, plating layer Zn percentage, and plating quantity perface of the surface-treated steel plate are not particularly limited.They may be selected from within proper ranges in consideration ofcorrosion resistance and mechanical strength. The plating layer Znpercentage is preferably 40% or greater, more preferably, 80% or greaterto improve corrosion resistance. In the case of, for example,Zn—Al—Mg-based alloy plating, a preferable Zn percent by weight is 80 orgreater in terms of corrosion resistance. If the Zn—Al—Mg-based alloyplating contains a large amount of Mg, the viscosity and surface tensionthereof decrease when it melts, to increase fluidity. This is preferablebecause it promotes a flow to a cut face.

If a surface-treated steel plate having a large plating quantity perface is selected, the laser beam cutting causes a larger amount ofplating metal to flow to a cut end face, thereby realizing a goodcorrosion resistance. Accordingly, the plating quantity per face on alaser beam irradiation side is preferably 20 g/m² or more, morepreferably, 30 g/m² or more or 90 g/m² or more.

As a ratio of the plating quantity per face to a plate thickness (aratio of plating quantity/plate thickness) increases, a ratio of aplating metal inflow coating layer to a cut end face increases.Accordingly, the ratio of a plating quantity per face (g/m²) to a platethickness (mm), i.e., the ratio of plating quantity/plate thickness ispreferably 1.3×10 or more, more preferably, 2.5×10 or more.

(Coating Layer)

A plating metal layer to be formed as a coating layer on a cut face of asurface-treated steel plate is satisfactory if it wholly or partlycovers the cut face. If the plating metal layer is partly present on thecut face, the plating metal on the cut face melts prior to a basematerial, i.e., steel of the cut face due to the sacrificial anodeeffect, thereby securing corrosion resistance for the cut face. Tosecure a satisfactory corrosion resistance, an average length of thecoating layer on the cut face is preferably 25% or more of the thicknessof the steel plate. In this specification, the length of the coatinglayer on a cut face is referred to as a “plating inflow length” and theratio of a coating layer average length to a steel plate thickness as a“plating inflow length ratio”.

This specification calls a coating layer occupying ratio on a cut face a“coverage”. The coverage is preferred to be 10% or more. If the coverageis less than 10%, the plating metal inflow is unable to secure asufficient corrosion resistance.

(Oxide Layer, Nitride Layer, or Mixed Layer of Them)

In laser cutting, a cutting gas is blown from around a laser beam towarda surface-treated steel plate. The cutting gas is mainly used to expelburned, evaporated, or melted material from a cut part. The cutting gasmay be an O₂ gas, air, an N₂ gas, or a mixed gas of them. An oxidelayer, a nitride layer, or a mixed layer thereof is formed on thesurface of a cut face that is exposed when cutting the surface-treatedsteel plate with a laser beam. At this time, a plating metal layer ofthe surface-treated steel plate flows to the cut face and forms theabove-mentioned coating layer. Accordingly, the coating layer is formedon the oxide layer, nitride layer, or mixed layer (hereinafter sometimesreferred to as an “oxide layer or the like”). If an auxiliary gas of thesame kind as the cutting gas is used, it will contribute to theformation of the oxide layer and the like.

The melted plating metal flowed to the cut face tends to spread and moveover the surface of the cut face. It is understood, therefore, that theoxide layer or the like acts to improve wettability between the meltedplating metal and the cut face. Accordingly, the cut face on which theoxide layer or the like is formed promotes the formation of the coatinglayer and increases the plating metal layer coverage.

(Laser Cutting Method)

The third embodiment is a laser cutting method for a surface-treatedsteel plate coated with a plating metal layer on the surface of a steelplate. The method conducts laser cutting with the use of a cutting gassuch as an oxygen gas, a nitrogen gas, or a mixed gas thereof to form acut face and jets an auxiliary gas toward a melted plating layer to makeit flow onto the cut face.

As mentioned above, a laser beam melts a plating metal layer on thesurface of a steel plate. At this time, the auxiliary gas is impingedtoward a peripheral area of the cut part, so that a flow of theauxiliary gas promotes the flowing of the melted plating metal towardthe cut face. A nozzle for jetting the auxiliary gas may be arrangedaround a nozzle for the cutting gas. FIG. 24(a) is a sectional diagramshowing a laser cutting nozzle as an example of such an arrangement.Schematically shown in the diagram is a relationship among an emittedlaser beam, a jetted cutting gas, and a jetted auxiliary gas. Around alaser beam emission nozzle 19, a cutting gas nozzle (cutting gas supplymeans 30) for jetting the cutting gas is arranged, and around thisnozzle, a nozzle (auxiliary gas supply means 40) for jetting theauxiliary gas is arranged. The cutting gas jetted from the cutting gassupply nozzle 30 acts on an area including the cut face 220. On theother hand, the auxiliary gas 70 jetted from the auxiliary gas supplynozzle 40 acts on a peripheral area around the cut part (cut face 220).FIG. 24(b) is a model diagram showing a pressure distribution of thecutting gas 60 and auxiliary gas 70 acting on the cut material 200. Asshown in FIG. 24(b), a predetermined pressure acts on the peripheralarea of the cut face 220.

FIGS. 25(a) and 25(b) are model diagrams showing the formation of acoating layer that contains the plating metal. As shown in FIG. 25(a),at the time of laser cutting, the auxiliary gas 70 is blown to themelted metal layer 230 on the surface of the steel plate 200. Then, asshown in FIG. 25(b), the melted plating metal layer 230 moves toward thecut face 220 and flows onto the cut face 220, thereby forming thecoating layer 250. Using also the auxiliary gas 70 efficiently promotesan inflow of the plating metal.

FIGS. 26(a), 26(b), 27(a), and 27(b) are diagrams schematically showingcutting states of a nozzle according to a related art employing only acutting gas. The cutting gas 60 is blown toward a cut part (cut face220) and part of the cutting gas forms a flow that diffuses around thecut part (FIG. 26(a)). However, a pressure of the cutting gas 60 actingon a melted plating metal layer is low (FIG. 26(b)). As a result, asshown in FIGS. 27(a) and 27(b), only evaporated plating metal 260 of themelted plating metal layer 230 is expelled. It is understood that themelted plating metal layer 230 never flows to the cut face.

The kind of an irradiation laser beam is not particularly limited. It ispossible to employ, for example, a CO₂ laser that oscillates a laserbeam of 3 μm or more in wavelength. Conditions of a laser beam at thetime of cutting such as a spot diameter, an output power, and a movingspeed are properly set according to the thickness, processing shape, andthe like of a surface-treated steel plate to be cut.

The plating metal layer of a surface-treated steel plate increases itstemperature when heated with a cutting laser and melts. Parametersaffecting a temperature increase of the plating metal layer are athickness (t: units of mm) of the surface-treated steel plate, a laseroutput (P: units of kW), a cutting speed (v: units of m/min), and alaser cutting width (w: units of mm). Even with the same laser output,the degree of temperature increase differs depending on the platethickness and cutting speed. Accordingly, to compare various heatingconditions of a plating metal layer with one another, an index of“P/v×t×w” is used. This index is a numeric value obtained by dividingthe laser output P (kW) by the cutting speed v (m/min), plate thicknesst (mm), and laser cutting width w (mm). This specification calls theindex a “laser heat gain index”. To make plating metal flow to a cut endface and form a proper coating layer, the laser heat gain index ispreferably in a range of 0.79 to 2.57. If the index is smaller than0.79, a heat gain at the time of cutting is too small, and therefore,dross deposits on a cut part to make the cutting impossible. On theother hand, if the index exceeds 2.57, the heat gain is too large, andtherefore, plating metal evaporates to reduce the quantity of platingmetal flowing to a cut end face, thereby deteriorating corrosionresistance of the cut end face.

The cutting gas for laser cutting is preferably an oxygen gas, anitrogen gas, or a mixed gas thereof to form a cut face having an oxidelayer, a nitride layer, or a mixed layer. The cutting gas may be an O₂gas, air, an N₂ gas, or a mixed gas thereof. The cutting gas may bemixed with an inert gas (for example, Ar). The flow rate and pressure ofthe cutting gas may properly be set according to the thickness andcutting conditions of a surface-treated steel plate.

The auxiliary gas supply means may be any means that jets the auxiliarygas 70 after laser cutting. An example configuration is to arrange,around the machining head 5 for jetting the cutting gas, a nozzle 80 forjetting the auxiliary gas 70. As shown in FIG. 28 , a plurality ofauxiliary gas side nozzles 80 may be arranged beside the machining head5. As shown in FIG. 29 , the machining head 5 may have an inner nozzle(not shown) for blowing off melted metal in a cut groove and anauxiliary gas nozzle 90 serving as an outer nozzle surrounding the innernozzle. The auxiliary gas nozzle 90 jets the auxiliary gas 70 toward themelted plating metal layer 230 to guide the same to the cut face 220.

Namely, the laser machining head adopted for the laser cutting andmachining method is preferably provided with a nozzle for jetting anassist gas toward a laser machining part of a plated steel plate to blowoff melted metal and form a cut face, as well as an auxiliary gas nozzlefor jetting an auxiliary gas to guide plating-layer-containing metalmelted at the top surface of the plated steel plate toward the cut face.This configuration guides, with the auxiliary gas, melted metal in arange of 0.27 mm to 0.5 mm from the cut face toward the cut face.

In the laser machining head, the auxiliary gas nozzle is preferablyconfigured to jet the auxiliary gas within a range larger than the widthof a laser cut-and-machined groove formed by laser cutting andmachining, i.e., within a range including the melted plating metal layer230.

The kind of the auxiliary gas is not particularly limited if it canpromote a flow of melted plating metal. A composition of the auxiliarygas may be similar to that of the cutting gas or may be an oxygen gas, anitrogen gas, or a mixed gas thereof. The composition of the auxiliarygas may differ from that of the cutting gas, or may be an inert gas (forexample, Ar) only.

A flow rate of the auxiliary gas can be set according to the thicknessof a surface-treated steel plate, a laser beam moving speed, and thelike. As mentioned above, the auxiliary gas has a function of promotinga flow of melted plating metal toward a cut face. The flow rate of theauxiliary gas is preferably 20 L/min or greater. If the flow rate of theauxiliary gas is small, the flow of plating metal toward a cut face willbe insufficient. If the flow rate is large, an inflow of melted platingmetal will increase. If the flow rate is excessively large, the meltedplating metal is excessively blown off, to prevent the formation of acoating layer, and therefore, it is not preferable.

Embodiment 2

Hereunder, an embodiment of the present invention will be explained. Thepresent invention is not limited to the following embodiment.

As surface-treated steel plates, steel plates having plating layers ofplating compositions shown in Tables 4 and 5 are used to prepare testpieces No. 1 to No. 47. The test piece No. 47 is a reference exampleaccording to plasma cutting employing air. The plating layers mentionedabove have a plating composition of Zn—Al—Mg, Zn—Al, Zn, or Al—Si. In acolumn “Plating composition” in the Tables 4 and 5, a test piecementioned as, for example, “Zn-6A1-3Mg” means a steel plate having aZn-based plating layer containing 6% Al in weight percent and 3% Mg inweight percent. As shown in the Tables 4 and 5, the test pieces havedifferent plating quantities (g/m²) per face, steel plate thicknesses(mm), and plating quantity to plate thickness ratios (ratios of platingquantity/plate thickness). In the Tables 4 and 5, the plating quantityper face is a value on a laser beam irradiating face.

Laser cutting is carried out by combining conditions mentioned below.

(a) Laser oscillation method: CO₂ laser

(b) Laser cutting width (mm): 0.24 to 0.40

(c) Laser output (kW): 2, 4, 6

(d) Cutting speed (m/min): 0.6 to 7.0

(e) Cutting gas kind: nitrogen (N₂), oxygen (O₂), nitrogen+3% oxygen(N₂+3% O₂), argon (Ar)

(f) Cutting gas pressure (MPa): 0.05 to 1.4

(g) Auxiliary gas nozzle type: side nozzle (A-type: refer to FIG. 28 ),ring nozzle (B-type: refer to FIG. 29 )

(h) Auxiliary gas kind: nitrogen (N₂), oxygen (O₂), nitrogen+3% oxygen(N₂+3% O₂), argon (Ar)

(i) Auxiliary gas flow rate (L/min): 15 to 1900

Test pieces after cutting are photographed on their cut faces to provideimage data and their coating layer average lengths (plating inflowlength ratios) and plating coverages are found. The test pieces aresubjected to an exposure test to be explained later, to find end facerusting ratios. In addition, the thicknesses of oxide layers and thelike of the test pieces are measured according to a method to beexplained later. The cut faces are subjected to a component analysis byelectron beam microanalyzer (EPMA).

(Coating Layer Average Length)

FIG. 30(a) schematically shows a method of measuring a coating layeraverage length. A coating layer shows, as indicated with plating inflows310, flows of plating metal coming from a steel plate surface 320 onto acut face and extending toward a steel plate bottom face 330. Asexemplary shown in FIG. 30(a), five plating inflows 310 encircled withcircular marks are selected as main inflows from within an observationarea, lengths (plating inflow lengths 340) thereof up to front endsthereof are measured, a ratio of each plating inflow length 340 to asteel plate thickness 350 (this specification calls the ratio the“plating inflow length ratio”) is calculated, and an average of the fivespots is calculated. According to the average, a coating layer averagelength of this embodiment is determined.

(Plating Coverage)

A method of measuring a coverage of a coating layer occupying a cut facewill be explained. First, as shown in FIG. 30(b), evaluation points P1to P5 are set. For this, the embodiment draws a vertical segmentperpendicularly to the surface of the steel plate, and on the segment,P1 and P5 are set at positions 50 μm from the top and bottom of a plateend face. At a midpoint between P1 and P5, P3 is set. At a mid pointbetween P1 and P3, P2 is set, and at a midpoint between P3 and P5, p4 isset. Among P1 to P5, points agreeing with the plating inflows 310 arecounted. As exemplary shown in FIG. 30(b), the same procedure isrepeated four times at optional locations to find points agreeing withthe plating inflows from among the total of 20 positions (points) and aratio thereof is calculated. For example, if there are eight agreeingpoints, the ratio is 8/20=0.4 (40%). This calculated value is used asthe plating coverage of the embodiment.

(Thicknesses of Oxide Layer and Other Layers)

A method of measuring the thicknesses of an oxide layer and other layerswill be explained. As shown in FIG. 31 (a), a test piece 390 with a cutface thereof being faced downward is buried in resin 400 to prepare ameasuring sample.

At this time, a wire 420 is arranged at an end of the test piece 390 toprovide the test piece 390 with an inclination angle θ. The test piece390 buried in resin is polished so that a plating layer, an oxide layer,and the like on the cut face of the test piece 390 are obliquelypolished to expose, on a polished surface 380, a steel plate basematerial 370, an oxide layer 360, and plating metal 310 side by side asshown in FIG. 31 (b). Thereafter, the widths of the oxide layer 360 andothers are measured. Based on the measured widths and the inclinationangle θ formed at the time of burying, the thicknesses of the oxidelayer and others are calculated. At optional three locations within anobservation area 410 on the cut face, the same procedure is taken tomeasure the thicknesses of the oxide layer and others and calculateaverages thereof. The averages are used as the thicknesses of the oxidelayer and others according to this embodiment.

(End Face Rusting Ratio)

In connection with the rust resistance of members processed according tothe present invention, the test pieces are subjected to an exposure testcarried out in the open air for 60 days and cut faces of the test piecesare evaluated according to red rust occurrence ratios. Thisspecification calls the red rust occurrence ratio the “end face rustingratio”. Hereunder, an end face rusting ratio measuring method will beexplained. At around a central part of the cut test piece, a measuringrange of 150 mm in length is set. As exemplarily shown in FIG. 32 ,judging positions 520 are set at 5-mm intervals in the measuring range,the number of the judging positions that cross red rust parts 510 aremeasured, and a crossing ratio is calculated. For example, in FIG. 32 ,there are twenty judging positions and seven of them cross the red rustparts. Therefore, the end face rusting ratio is calculated as 7/20=0.35(35%).

Measured results of the plating inflow length ratio, plating coverage,oxide layer and other thicknesses, and end face rusting ratio are shownin Tables 4 and 5.

TABLE 4 Ratio of plating quantity Plate (g/m2)/ Heat gain thik- Platingplate Laser Cutting Laser Cutting index ness Plating quantity thick-oscillation width output speed P/v × Kind of No t(mm) composition g/m2ness(mm) method w (mm) P (kW) v (m/min) t × w cutting gas 1 0.6Zn—6Al—3Mg 60 100.0 CO2 0.24 2 7.0 1.98 N2 2 1.6 Zn—6Al—3Mg 90 56.3 CO20.24 2 4.0 1.30 N2 3 2.3 Zn—6Al—3Mg 90 39.1 CO2 0.26 4 2.6 2.57 N2 4 2.3Zn—6Al—3Mg 190 82.6 CO2 0.26 4 2.6 2.57 N2 + 3% O2 5 2.3 Zn—6Al—3Mg 19082.6 CO2 0.26 4 2.6 2.57 N2 + 3% O2 6 3.2 Zn—6Al—3Mg 45 14.1 CO2 0.3 42.0 2.08 N2 7 3.2 Zn—6Al—3Mg 90 28.1 CO2 0.3 4 3.4 1.23 O2 8 3.2Zn—6Al—3Mg 190 59.4 CO2 0.3 4 1.9 2.19 N2 9 3.2 Zn—6Al—3Mg 300 93.8 CO20.3 4 1.9 2.19 N2 + 3% O2 10 3.2 Zn—6Al—3Mg 45 14.1 CO2 0.3 4 2.0 2.08N2 11 3.2 Zn—6Al—3Mg 90 28.1 CO2 0.3 4 2.0 2.08 O2 12 6.0 Zn—6Al—3Mg 9015.0 CO2 0.4 4 2.0 0.83 O2 13 6.0 Zn—6Al—3Mg 190 31.7 CO2 0.36 4 2.00.93 N2 14 6.0 Zn—6Al—3Mg 300 50.0 CO2 0.36 4 1.5 1.23 N2 + 3% O2 15 6.0Zn—6Al—3Mg 190 31.7 CO2 0.36 4 1.5 1.23 N2 16 9.0 Zn—6Al—3Mg 120 13.3CO2 0.4 6 2.1 0.79 O2 17 9.0 Zn—6Al—3Mg 190 21.1 CO2 0.36 6 1.8 1.03 N218 9.0 Zn—6Al—3Mg 190 21.1 CO2 0.4 6 2.1 0.79 O2 19 3.2 Zn—11Al—3Mg 9028.1 CO2 0.3 4 1.9 2.19 N2 + 3% O2 20 3.2 Zn—11Al—3Mg 190 59.4 CO2 0.3 41.9 2.19 N2 + 3% O2 21 3.2 Zn—2.5Al—3Mg 90 28.1 CO2 0.3 4 1.9 2.19 N2 +3% O2 22 3.2 Zn—4Al—0.7Mg 90 28.1 CO2 0.3 4 1.9 2.19 N2 + 3% O2 23 3.2Zn—4Al—0.7Mg 190 59.4 CO2 0.3 4 1.9 2.19 N2 + 3% O2 24 3.2 Zn—3.5Al—3Mg90 28.1 CO2 0.3 4 1.9 2.19 N2 + 3% O2 25 3.2 Zn—5Al 90 28.1 CO2 0.3 41.9 2.19 N2 + 3% O2 26 3.2 Zn—55Al 90 28.1 CO2 0.3 4 1.9 2.19 N2 + 3% O227 3.2 Zn—55Al 190 59.4 CO2 0.3 4 1.9 2.19 N2 + 3% O2 28 3.2 Zn 90 28.1CO2 0.3 4 1.9 2.19 N2 + 3% O2 29 3.2 Zn 190 59.4 CO2 0.3 4 1.9 2.19 N2 +3% O2 Plating Oxide/ inflow nitride Auxiliary length layer Cutting gasAuxiliary gas flow ratio Plating thick- End face pressure nozzleAuxiliary rate length coverage ness rusting No Mpa type gas L/min ratio:% % μm ratio % 1 0.5 A N2 150 73 50 0.1 0 Embodiment 2 0.8 A N2 50 60 250.1 5 Embodiment 3 0.8 B N2 200 52 20 0.1 5 Embodiment 4 0.9 B N2 + 3%O2 200 54 25 0.6 0 Embodiment 5 0.9 B N2 + 3% O2 200 48 20 0.5 0Embodiment 6 0.8 A N2 1000 32 10 0.1 5 Embodiment 7 0.05 A O2 25 44 150.7 0 Embodiment 8 0.8 B N2 800 55 20 0.1 0 bmbodiment 9 0.9 B N2 + 3%O2 700 73 30 0.6 0 Embodiment 10 0.8 A N2 1000 32 10 0.2 5 Embodiment 110.05 A 02 25 39 15 0.6 0 Embodiment 12 0.07 A 02 50 37 10 0.7 10Embodiment 13 1.0 A N2 1500 43 30 0.1 0 Embodiment 14 1.0 A N2 + 3% O21500 43 25 0.7 0 Embodiment 15 1.0 A Ar 1500 41 25 0.1 0 Embodiment 160.08 B O2 75 33 10 0.6 5 Embodiment 17 1.4 B N2 1900 44 20 0.1 0Embodiment 18 0.08 B Ar 75 26 10 0.6 5 Embodiment 19 0.9 A N2 + 3% O2700 40 15 0.8 0 Embodiment 20 0.9 A N2 + 3% O2 700 52 15 0.7 0Embodiment 21 0.9 A N2 + 3% O2 700 35 10 0.6 0 Embodiment 22 0.9 A N2 +3% O2 700 32 10 0.6 5 Embodiment 23 0.9 A N2 + 3% O2 700 34 15 0.5 0Embodiment 24 0.9 A N2 + 3% O2 700 31 15 0.8 0 Embodiment 25 0.9 B N2 +3% O2 700 33 10 0.6 5 Embodiment 26 0.9 B N2 + 3% O2 700 28 10 0.6 0Embodiment 27 0.9 B N2 + 3% O2 700 30 15 0.5 0 Embodiment 28 0.9 B N2 +3% O2 700 31 15 0.6 0 Embodiment 29 0.9 B N2 + 3% O2 700 33 20 0.7 0Embodiment

TABLE 5 Ratio of plating quantity Plate (g/m2) Heat gain thik- Platingplate Laser Cutting Laser Cutting index ness Plating quantity thick-oscillation width output speed P/v × Kind of No t(mm) composition g/m2ness(mm) method w (mm) P (kW) v (m/min) t × w cutting gas 30 0.6Zn—6Al—3Mg 60 100.0 CO2 0.24 2 7.0 1.98 N2 31 2.3 Zn—6Al—3Mg 90 39.1 CO20.26 4 2.7 2.48 N2 32 2.3 Zn—6Al—3Mg 190 82.6 CO2 0.26 4 2.4 2.79 N2 +3% O2 33 3.2 Zn—6Al—3Mg 90 28.1 CO2 0.38 4 4.5 0.73 O2 34 3.2 Zn—6Al—3Mg300 93.8 CO2 0.3 4 1.9 2.19 N2 + 3% O2 35 6.0 Zn—6Al—3Mg 300 50.0 CO20.36 4 1.5 1.23 N2 + 3% O2 36 3.2 Zn—11Al—3Mg 190 59.4 CO2 0.3 4 1.92.19 N2 + 3% O2 37 3.2 Zn—55Al 190 59.4 CO2 0.3 4 1.9 2.19 N2 + 3% O2 383.2 Zn 190 59.4 CO2 0.3 4 1.9 2.19 N2 + 3% O2 39 2.3 Zn—6Al—3Mg 190 82.6CO2 0.26 4 1.9 3.52 N2 + 3% O2 40 3.2 Zn—6Al—3Mg 300 93.8 CO2 0.3 4 1.62.60 N2 + 3% O2 41 3.2 Zn—6Al—3Mg 300 93.8 CO2 0.3 4 6.1 0.68 N2 + 3% O242 6.0 Zn—6Al—3Mg 300 50.0 CO2 0.36 4 0.6 3.09 N2 + 3% O2 43 2.3Zn—6Al—3Mg 190 82.6 CO2 0.26 4 2.7 2.48 N2 + 3% O2 44 3.2 Zn—6Al—3Mg 309.4 CO2 0.3 4 3 1.39 N2 45 3.2 Zn—5Al—3Mg 190 59.4 CO2 0.3 4 2.0 2.08 Ar46 2.3 Al—10Si 60 26.1 CO2 0.26 4 2.7 2.48 N2 + 3% O2 47 3.2 Zn—6Al—3Mg190 59.4 Plasma Plating inflow Oxide/ Auxiliary length nitride Cuttinggas Auxiliary gas flow ratio Plating layer End face pressure NozzleAuxiliary rate length coverage thick- rusting No Mpa type gas L/minratio: % % ness μm ratio % 30 0.5 None 8 5 0.1 35 Comparison 31 0.8 None12 5 0.1 30 Comparison 32 0.8 None 13 10 0.6 25 Comparison 33 0.05 None11 0 0.7 45 Comparison 34 0.9 None 23 10 0.6 25 Comparison 35 1.0 None21 5 0.7 35 Comparison 36 0.9 None 5 5 0.7 35 Comparison 37 0.9 None 105 0.6 40 Comparison 38 0.9 None 6 0 0.7 45 Comparison 39 0.9 B N2 + 3%O2 200 3 0 2.4 45 Comparison 40 0.9 B N2 + 3% O2 700 6 5 0.6 65Comparison 41 0.9 B N2 + 3% O2 700 Defective cut Comparison 42 1.0 BN2 + 3% O2 1500 9 0 2.7 55 Comparison 43 0.9 B N2 + 3% O2 15 4 0 0.6 45Comparison 44 0.6 B N2 1000 0 0 0.1 50 Comparison 45 0.8 B Ar 1000 18 100 20 Comparison 46 0.9 B N2 + 3% O2 200 31 15 0.6 85 Comparison 47 0 0 3100 Comparison

According to the EPMA analysis results, the test pieces No. 1 to No. 29processed with the auxiliary gas each show a Zn component detected on acut face. The Zn component is distributed like flows passing from thetop surface of the plate irradiated with a laser beam to the bottom ofthe plate (FIG. 33 ). Based on this distribution state, it is presumedthat the Zn component on the cut face is derived from the plating metallayer that has flowed from the surface of the steel plate onto the cutface.

According to the analysis results of oxygen and nitride components, thecut faces of the test pieces No. 1, No. 2, and the like employing an N₂cutting gas each have a nitride layer in an area where no Zn componentis present. The cut faces of the test pieces No. 4, No. 5, and the likeemploying a mixed gas containing N₂ gas and 3% O₂ and the cut faces ofthe test pieces NO. 7, No. 11, and the like employing an O₂ gas eachhave an oxide layer, a nitride layer, or a mixed layer thereof in anarea where no Zn component is present.

According to these analysis results, it is presumed that an oxide layeror the like is formed on a cut face after laser cutting, and thereafter,a melted plating metal layer on the surface of a steel plate flows ontothe cut face to form a coating layer over the oxide layer or the like.

As shown in the Table 4, the test pieces No. 1 to No. 29 correspondingto the embodiment of the present invention each have a plating inflowlength ratio of 25% or more, a cut face coverage of 10% or more, and anoxide layer or the like under the coating layer. The test pieces No. 1to No. 29 each show an end face rusting ratio of 10% or smaller toindicate a good end face rust resistance. Also, the test pieces No. 1 toNo. 29 each have a Zn containing ratio of 40% or more in the platingmetal layer and a plating quantity per face of 20 g/m² or more and eachshow 0.1 μm or more in the average thickness of an oxide layer or thelike and 1.3×10 or more in the ratio of plating quantity (g/m²) to steelplate thickness (mm).

In addition, the laser cutting method of the test pieces No. 1 to No. 29employs a cutting gas containing an oxygen gas, a nitrogen gas, or amixed gas thereof, as well as an auxiliary gas. The auxiliary gas is anargon gas for the test piece No. 15 and an oxygen gas, a nitrogen gas,or a mixed gas thereof for the other. An auxiliary gas nozzle used isthe side nozzle (A) or the ring nozzle (B). The laser cutting is carriedout with laser heat gain indexes (P/v×t×w) within a range of 0.79 to2.57.

On the other hand, the comparative test pieces No. 30 to No. 45 shown inthe Table 5 each employ, similar to the embodiment of the presentinvention shown in Table 4, a surface-treated steel plate having aplating metal layer containing Zn. Their plating inflow length ratioseach are lower than 25% and their end face rusting ratios each exceed10%, to show an inferior corrosion resistance compared to the embodimentof the present invention.

Among the comparative examples, the test pieces No. 30 to No. 38 areexamples each employing no auxiliary gas. The test pieces No. 39 to No.42 are examples each employing an auxiliary gas and a laser heat gainindex (P/v×t×w) out of the range of 0.79 to 2.57. The test piece No. 41employs a laser heat gain index below 0.79, and due to a lack of heatamount, the cutting thereof is impossible. The test piece No. 43 employsan auxiliary gas flow rate below 20 L/min. The test piece No. 44 employsa ratio of plating quantity/plate thickness less than 1.3×10. The testpiece No. 45 employs an argon gas as a cutting gas, to form no oxide ornitride layer.

The test piece No. 46 shown in the Table 5 is a comparative exampleemploying plating metal (Al—Si) containing no Zn. The test piece No. 47is a comparative example employing plasma cutting instead of lasercutting. Each of them has an end face rusting ratio far greater than10%, to demonstrate an inferior end face corrosion resistance.

According to the above-mentioned test results, it is confirmed that amember having factors specific to the present invention demonstrates agood end face corrosion resistance.

INDUSTRIAL APPLICABILITY

The present invention is capable of carrying out laser cutting andmachining without removing plating of a plated steel plate. When lasercutting a plated steel plate, the present invention is able to makemelted and/or evaporated plating-layer-containing metal on the topsurface of the plated steel plate flow to a cut face and coat the cutface. Accordingly, the present invention is able to efficiently lasercut and machine a plated steel plate, and after the laser cutting andmachining, requires no rustproofing process to be carried out again onthe cut face.

What is claimed is:
 1. A laser cut-and-machined product made from aplated steel plate, the laser cut-and machined product comprising: aplating metal layer formed on a top surface of the plated steel plate; acut face formed on the plated steel plate; and a coating layer that isformed on the cut face from a material of the plating metal layer in aregion around the cut face that is melted and/or evaporated at the timeof laser cutting and machining.
 2. The laser cut-and-machined productaccording to claim 1, wherein a plating thickness of the plating metallayer around an upper edge of the cut face is thinner than a platingthickness of the plating metal layer at a position away from the cutface.
 3. The laser cut-and-machined product according to claim 1,wherein a plating melting range is within a range of 0.27 mm to 0.5 mmfrom the cut face.
 4. A surface-treated steel plate, comprising: aplating metal layer formed on a top surface of the surface-treated steelplate; wherein a cut face formed on the surface- treated steel plate;and a coating layer that is formed on the cut face from a material ofthe plating metal layer in a region around the cut face.
 5. Thesurface-treated steel plate according to claim 4, wherein an area of thecoating layer that covers the cut face is 10% or more of an area of thecut face, or the coating layer covers the cut face by an extent of 30%or more of a plate thickness from a top or bottom surface of thesurface-treated steel plate.